The Philippines sits on the Pacific Ring of Fire — one of the most seismically active regions in the world. Every engineer practicing in the Philippines must be competent in computing seismic base shear per NSCP 2015 Section 208.
This guide covers the complete static force procedure: seismic zone classification, soil profile, importance factors, response modification factors, and the base shear formula.
Based on NSCP 2015 (7th Edition), Section 208 — Earthquake Loads. The static force procedure applies to regular structures not exceeding 73m in height. This guide is part of AEDO's NSCP 2015 Complete Guide →
NSCP 2015 divides the Philippines into Seismic Zone 2 and Zone 4 (Zone 3 is skipped to align with older UBC precedent):
| Seismic Zone | Zone Factor Z | Areas |
|---|---|---|
| Zone 2 | 0.20 | Parts of western Mindanao (Zamboanga peninsula) |
| Zone 4 | 0.40 | Most of the Philippines including Metro Manila, Visayas, most of Mindanao |
Soil profile affects how seismic waves amplify through the ground to your structure. NSCP 2015 Table 208-2 defines six soil profile types — use the free soil profile classifier to get your site's type from Vs, N-value, or su:
| Profile | Description | Typical Condition |
|---|---|---|
| SA | Hard rock | Unweathered rock, Vs > 1500 m/s |
| SB | Rock | Vs = 760–1500 m/s |
| SC | Very dense soil / soft rock | Vs = 360–760 m/s |
| SD | Stiff soil | Vs = 180–360 m/s (most common in PH) |
| SE | Soft soil | Vs < 180 m/s |
| SF | Special soils | Liquefiable, sensitive, organic — requires site-specific study |
Ca and Cv are acceleration and velocity seismic response coefficients from NSCP Tables 208-7 and 208-8, based on Zone and Soil Profile:
| Soil Profile | Ca (Zone 4) | Cv (Zone 4) |
|---|---|---|
| SA | 0.32 | 0.32 |
| SB | 0.40 | 0.40 |
| SC | 0.40 | 0.56 |
| SD | 0.44 | 0.64 |
| SE | 0.56 | 0.96 |
| Occupancy Category | I | Examples |
|---|---|---|
| Standard | 1.0 | Residential, commercial |
| Essential | 1.25 | Schools, assembly halls |
| Critical | 1.50 | Hospitals, emergency facilities |
R reflects the ductility and energy dissipation capacity of the structural system per NSCP Table 208-11:
| Structural System | R |
|---|---|
| Special Moment Resisting Frame (SMRF) — concrete | 8.5 |
| Intermediate Moment Resisting Frame (IMRF) — concrete | 5.5 |
| Ordinary Moment Resisting Frame (OMRF) — concrete | 3.5 |
| Special Steel Moment Frame | 8.5 |
| Shear Wall — concrete | 4.5 |
Use the approximate method per NSCP 208.5.2.2:
T = Ct × hn^(3/4)
Ct = 0.0853 (steel MRF) · 0.0731 (concrete MRF) · 0.0488 (other) · hn = height in meters
V = (Cv × I × W) / (R × T)
Subject to: Vmax = (2.5 × Ca × I × W) / R · Vmin = 0.11 × Ca × I × W · Zone 4 min = 0.8 × Z × Nv × I × W / R
W = total seismic dead load (all dead loads + applicable live load fractions per NSCP 208.5.3)
Distribute V over the building height per NSCP 208.5.5. For structures with T > 0.7s, an additional top force Ft = 0.07TV is applied at the roof before distributing the remainder proportionally to floor weights and heights.
The base shear you just computed is not an abstract number — it is a real horizontal force the structure must carry from the top of the building all the way into the soil. NSCP §208 requires a complete, unbroken lateral-force-resisting system: every floor diaphragm, frame, shear wall, and footing is a link in that chain. The diagram below traces that path.
The BuildX NSCP Kit app handles the full Section 208 seismic procedure — zone selection, soil profile, Ca/Cv tables, period calculation, base shear, and vertical distribution.
AEDO Construction provides full structural engineering services for residential and commercial buildings in the Philippines — NSCP 2015 compliant seismic and wind design, structural plans, BOQ, and design-build construction.